1. Field of the Invention
The present invention relates to a light emitting device that can be used in a liquid crystal backlight, an illumination light source, and various types of indicators, displays and traffic signal lights, and relates particularly to a light emitting device in which the light emitting element is coated with a silicone resin.
2. Description of the Prior Art
In light emitting devices that use a light emitting element such as an LED, the light emitting element is typically coated with a phosphor-containing layer, or a transparent resin layer that functions as a lens or the like. Conventionally, epoxy resins have been used as the material for these coating layers, but recently, silicone resins have begun to attract attention, as a result of their superior levels of heat resistance.
However, with short wavelength LEDs such as blue LEDs and ultraviolet LEDs now being developed, the resin layer now requires a tougher material that is not only capable of withstanding heat generation, but is also able to withstand this type of high energy, short wavelength light. Previously proposed silicone resins have been addition curable resins that use a hydrosilylation reaction, meaning the proportion of silethylene linkages within the cured product is high. Because silethylene linkages are prone to cleavage by light or heat, the main skeleton within the cured product is prone to deterioration, and the resin is prone to bleed-out of low molecular weight, fluid, oily silicone. As a result, the mechanical strength of the cured product falls, and the resin is more likely to become brittle and prone to heat deformation. Furthermore, the low molecular weight silicone components that bleed out can cause a variety of faults. In addition, if the resin reaches this state, then color irregularities and tone variation are more likely to occur during light emission, meaning the color tone characteristics of the light emitting element may be affected.
Conventionally, application of a sealing resin to a light emitting element has been conducted using a method in which the resin is dripped onto the light emitting element. However, the uniformity of the film thickness of a resin layer formed using this method is poor, which can cause color irregularities during light emission. In order to overcome the drawbacks associated with this type of dripping method, screen printing methods are now starting to be used. In a screen printing method, the resin used should exhibit ready separation from the metal mask during printing, and superior adhesion to the light emitting element following curing, but a resin material that adequately satisfies these requirements is, as yet, still unknown.